The present invention relates to a self-standing parallel plate beam splitter, a method for manufacturing the same, and a laser diode package structure using the same, and, more particularly, to a method for manufacturing a beam splitter that reflects or transmits light depending upon wavelengths of the light in the form of a self-standing parallel plate, a bidirectional-communication laser diode package structure manufactured using such a self-standing parallel plate beam splitter, and a laser diode package structure having triplexer and wavelength locking functions.
In recent years, optical communication using light as an information communication medium for large-capacity information transmission and high-speed information communication has been generalized. Consequently, it is possible to easily convert an electric signal of 10 Gbps (giga bit per sec) into laser light using a semiconductor laser diode chip having a width and length of approximately 0.3 mm. Also, it is possible to easily convert an optical signal transmitted through an optical fiber into an electric signal using a semiconductor light receiving device. Light is an energy wave exhibiting a very peculiar property. It is required for various lights simultaneously existing at a certain region to have the same wavelength, the same phase, and the same advance direction in order that the lights interact with one another. Consequently, the interference between lights is very low. For this reason, it is preferred to use a wavelength division multiplexing (WDM) type optical communication for simultaneously transmitting lights having different wavelengths through a single optical fiber using such a property of the lights. In the WDM type optical communication, the optical fiber, which is a signal transmission medium, is shared, with the result that optical fiber installation costs are reduced. Consequently, the WDM type optical communication is a very economical communication method. The WDM method is a technology for transmitting and receiving laser lights having different wavelengths through a single optical fiber. The WDM method is being commercialized in several communication fields now because the WDM method maximizes the capacity of information transmission using a single optical fiber. The WDM method is classified as a coarse WDM (CWDM) method, which is used when the difference between the wavelengths of the laser lights is great, or a dense WDM (DWDM).
The CWDM method may be classified as a communication method using laser light having a 1300 nm (nano meter) wavelength band and laser light having a 1490 nm or 1550 nm wavelength band or as a communication method simultaneously using all the wavelengths, i.e., 1300 nm, 1490 nm, and 1550 nm. In recent years, a fiber to the home (FTTH) method for connecting an optical fiber to the inside of a home of a communication subscriber has been increasingly generalized. The FTTH method, in which the optical fiber is drawn to the inside of the home of the communication subscriber to perform optical communication, needs an upstream optical communication mode, in which an optical signal is generated in the home of the communication subscriber and the generated optical signal is transmitted to a base station for optical communication, and a downstream optical communication mode, in which an optical signal transmitted from the base station for optical communication is converted into an electric signal. There has been proposed an optical communication method using an optical fiber for processing an upstream optical communication signal and an optical fiber for processing a downstream optical communication signal, wherein the optical fiber for processing the upstream optical communication signal and the optical fiber for processing the downstream optical communication signal are separately installed. However, this method brings about waste of optical fibers.
In recent years, therefore, a bidirectional optical communication method for transmitting an upstream optical signal and a downstream optical signal using a single optical fiber has been widely adopted. A light receiving device for receiving an optical signal downstream-transmitted through the optical fiber and converting the received optical signal into an electric signal and a light transmitting device for converting an electric signal into an optical signal and transmitting the converted optical signal through the optical fiber are integrated to manufacture a module that can be optically coupled to a single optical fiber, which is commonly referred to as a bidirectional (BiDi) module. For the BiDi module, it is required that a light transmitter and a light receiver of the BiDi module be simultaneously optically connected to a single optical fiber. Consequently, the BiDi module needs a function to adjust the advance direction of light depending upon the wavelength of the light.
FIG. 1 is a schematic view illustrating a typical BiDi module that is presently commercialized. The BiDi module is manufactured based on a transistor outline (hereinafter, referred to as a “TO”) type laser diode module and a TO type light receiving module. In the structure as shown in FIG. 1, the advance direction of an optical signal of a wavelength emitted from an optical fiber 2 is changed 90 degrees by a 45-degree filter 3 having wavelength selectivity, and is then incident on a TO type light receiving device 5, disposed below the 45-degree filter 3, with the result that the optical signal is converted into a current signal. Laser light of a wavelength emitted from a TO type laser diode module 4 is transmitted through the 45-degree filter 3 and is then condensed to the optical fiber 2. A filter having a function to reflect or transmit light, depending upon the wavelength of the light, to divide the light is called a beam splitter. In the BiDi module, i.e., the module for bidirectional optical communication, therefore, the beam splitter, which changes the advance path of light depending upon the wavelength of the light, plays an important role. On the beam splitter is deposited a material exhibiting a property to transmit or reflect light depending upon the wavelength of the light. However, the conventional BiDi module needs two TO type optical modules and a beam splitter to separately process the upstream optical signal and the downstream optical signal. In addition, a housing 1 is further needed to fix the TO type optical modules and the beam splitter. As a result, the structure of the BiDi module is complicated, and a large number of parts are needed to manufacture the BiDi module, with the result that the manufacturing costs of the BiDi module increase.
Also, the installation angle of the beam splitter may deviated from a desired angle when the beam splitter, the TO type laser diode module, and the TO type light receiving device are assembled. When the installation angle of the beam splitter may deviated from the desired angle, the position of light, which is incident from the optical fiber, is reflected by the beam splitter, and is then incident on a photo diode chip installed in the TO type light receiving device, changes. The position of light incident on the photo diode chip is decided based on the accuracy in installation angle of the beam splitter and the distance between the beam splitter and the photo diode chip. When the distance between the beam splitter and the photo diode chip is small, the alignment error of the beam splitter decreases, and the position error of light incident on the photo diode chip decreases, with the result that the positional alignment of the photo diode chip is easily carried out. Consequently, decreasing the distance between the beam splitter and the photo diode chip is an important factor that easily increases assembly accuracy and thus improves productivity. In the above, decreasing the distance between the beam splitter and the photo diode chip was discussed. However, this discussion may also be applied to decrease the distance between the beam splitter and a laser diode chip installed in the laser diode module. Therefore, decreasing the distance between the beam splitter and the laser diode chip and the distance between the beam splitter and the photo diode chip is a method for increasing margins on alignment errors of the respective optical parts, in addition to the reduction of a space, and therefore, improving assembly productivity. Consequently, a possible method for minimizing the distance between the beam splitter and the laser diode chip or the distance between the beam splitter and the photo diode chip will be a very important technical factor that greatly improves productivity.
Meanwhile, there have been proposed several methods for assembling the BiDi module using a method for reflecting laser light emitted from the laser diode module at the incline plane of the beam splitter to change the advance direction of the laser light and transmitting laser light for reception through the incline plane of the beam splitter.
FIG. 2 is a conceptional view of U.S. Pat. No. 4,733,067 to which one of such assembling methods is applied. The U.S. Pat. No. 4,733,067 shown in FIG. 2 discloses a module including a prism as the beam splitter and constructed in a structure in which the prism changes the advance direction of laser light horizontally incident on the side thereof by 90 degrees, and the received light passes through the prism such that the light is received by a receiver below the prism. When the prism is used as the beam splitter, as shown in FIG. 2, it is required to deposit a coating layer exhibiting a reflection or transmission property on the bottom plane and the incline plane of the prism, which are not parallel to each other but form an angle of 45 degrees. However, the deposition process is complicated, and therefore, the manufacturing costs of the module increase. Also, it is required to polish the bottom plane and the incline plane of the prism, which form an angle of 45 degrees, such that the bottom plane and the incline plane become mirror planes, during the manufacture of the prism. However, the polishing process is performed after the completion of a form of the prism, with the result that the manufacturing costs increase. Also, in FIG. 2, light advancing perpendicularly downward and transmitted through the incline plane of the prism does not advance perpendicularly downward after being transmitted through the bottom plane of the prism, with the result that accurate assembly is difficult. FIG. 3 is a view illustrating an example of a light advance path exhibiting the property of such a prism. Here, it is assumed that the prism is made of a glass material exhibiting a refractive index of 1.5.
As shown in FIG. 3, light incident perpendicularly downward on the incline plane of the prism advances in the prism at an angle of 28 degrees to the perpendicular direction according to Snell's law, exits from the bottom plane of the prism at an angle of 26 degrees to the perpendicular direction, and then continues to advance. This shows that it is required for the position of an active region to be changed depending upon the distance between the prism and the light receiving device, i.e., the photo diode chip, which acts as a factor that makes it difficult to arrange the photo diode chip.
FIG. 4 is a conceptional view of U.S. Pat. No. 7,093,988 to which a parallel plate beam splitter is applied. As shown in FIG. 4, U.S. Pat. No. 7,093,988 discloses a structure in which at a parallel plate beam splitter is installed at a support member having a tilt angle of 45 degrees, a laser diode chip is installed at one side of the beam splitter, and a photo diode chip is installed below the beam splitter. In this patent, the support member having the tilt angle of 45 degrees is needed, and the beam splitter is installed at the support member, with the result that the volume of the module increases, and the assembly process is complicated, whereby the manufacturing costs of the module increase.
FIG. 5 is a conceptional view of U.S. Pat. No. 4,807,238 using a silicon substrate. As shown in FIG. 5, this U.S. patent discloses a method for changing the advance direction of laser light emitted horizontally from a horizontally-disposed laser diode chip upward using the etched incline plane of the silicon substrate. For easy understanding, the plane direction of a crystal will be indicated hereinafter using braces. In this patent, when a wafer constructed in a diamond structure, such as silicon, having a plane direction of {100} plane is etched with an anisotropic etching solution, the etched side becomes {111} plane. The {111} plane has a tilt angle of 54.74 degrees to the {100} bottom plane. Consequently, light emitted horizontally from the laser diode chip is reflected at the incline plane, with the result that the light is emitted at a tilt angle, not in the perpendicular direction. In order to change the optical axis, which is tilted, of the laser light advancing upward into the perpendicular direction, it is required to install a Fresnel lens on the incline plane. However, the Fresnel lens is a part that is very difficult to manufacture, and therefore, the manufacturing costs of the Fresnel lens are high. Furthermore, it is required to accurately adjust the optical axis of the laser light, the advance direction of which is changed at the incline plane and the Fresnel lens. However, this process is difficult and troublesome.
FIG. 6 is a conceptional view of U.S. Pat. No. 5,566,265 using a trapezoidal beam splitter. This U.S. patent discloses a single TO type package that is capable of performing bidirectional communication using a trapezoidal beam splitter constructed in a structure in which one side of the beam splitter has a tilt angle of approximately 45 degrees and the other side of the beam splitter is vertical. In the trapezoidal beam splitter of this U.S. patent, light received from an optical fiber is incident on the incline plane, the tilt angle of which is approximately 45 degrees, and then exits from the horizontal plane. Consequently, as previously described in connection with FIG. 3, the incident direction of the light before the light is incident on the beam splitter is different from the advance direction of the light after the light exits the beam splitter, with the result that the assembly is very difficult. Furthermore, the trapezoidal beam splitter applied to this U.S. patent transmits or reflects light through the direct use of the incline plane. Consequently, it is required to form a coating layer on the incline plane by deposition; however, the accurate deposition of the thin film on the incline plane is a very difficult process, and therefore, it is difficult to manufacture the beam splitter.
FIG. 7 is a conceptional view of U.S. Pat. No. 4,733,067 using two prisms. This U.S. patent discloses a module that is capable of performing bidirectional communication in the form of a single TO type package using a beam splitter constructed in a structure in which two prisms are attached to each other. In the beam splitter, light is transmitted through or reflected at three planes of each prism, i.e., a total of six planes of the two prisms. However, a process for forming coating layers at the six planes of the two prisms by deposition and attaching the two prisms, three planes of each of which are coated, to complete the beam splitter is very complicated. Furthermore, the manufacturing costs of the beam splitter are very high.
FIG. 8 is a conceptional view of U.S. Pat. No. 6,879,784 using a parallelogram-shaped beam splitter. The U.S. patent shown in FIG. 8 discloses a method of dividing light, by an optical pick-up, using a beam splitter implemented by a parallelogram-shaped prism. Light is transmitted through or reflected at four planes of the parallelogram. Consequently, it is required to form the four planes of the parallelogram to be very smooth planes, such as mirror planes. Also, it is required to deposit dielectric thin film on the four planes of the parallelogram such that the four planes have appropriate reflection and transmission properties. As a result, the manufacture of the beam splitter is very difficult. Furthermore, laser light emitted from a laser diode chip, installed in the lower part of the module, is incident on the prism at an incident angle of 45 degrees and then exits from a horizontal plane having an angle of 45 degrees to an incline plane of 45 degrees. As previously in connection with FIG. 3, therefore, the optical axis of the light after being transmitted through the prism does not have an upward perpendicular direction, with the result that the optical axis alignment is difficult.
As can be clearly understood from the above description, the conventional BiDi module for bidirectional communication has problems in that the manufacture of the beam splitter is complicated, and the assembly for optical alignment is difficult, whereby manufacturing costs of the module greatly increase.
Meanwhile, there is a growing worldwide tendency, in recent optical communication, to need a triple play to combine data, voice, and analog video (broadcasting) into one. ITU.T G983.3, an international standard for communication, assigns frequency such that a wavelength of 1260 to 1360 nm (hereinafter, referred to as a 1310 nm wavelength band) is assigned as an upstream data signal, a wavelength of 1480 to 1500 nm (hereinafter, referred to as a 1490 nm wavelength band) is assigned as a downstream data signal, and a wavelength of 1550 to 1560 nm (hereinafter, referred to as a 1550 nm wavelength band) is assigned as a downstream video signal. Accordingly, an optical module is needed which is capable of upstream-transmitting an optical signal of a 1310 nm wavelength band and simultaneously transmitting and receiving a downstream data signal of a 1490 nm wavelength band and a downstream video signal of a 1550 nm wavelength band through a single optical fiber. Such an optical module having the above-specified functions is called a triplexer module. In the triplexer module, a function to change the advance path of light and divide wavelength is required to simultaneously optically connect a single optical fiber, a light transmitter, and a plurality of light receivers to one another.
FIG. 9 is a conceptional view of U.S. Pat. No. 6,493,121 to which such as triplexer module is applied.
The U.S. patent shown in FIG. 9 discloses a method for transmitting three kinds of optical signals through a single optical fiber using three TO type laser diode modules, a photo diode module for a 1490 nm band, and a photo diode module for a 1550 nm band. In the conventional triplexer structure shown in FIG. 9, an optical signal of a 1550 nm band and an optical signal of a 1490 nm band, transmitted while being mixed through the optical fiber, are reflected 90 degrees at a first beam splitter, with the result that the optical signal of the 1550 nm band is incident on the photo diode module for 1550 nm band reception, whereby the optical signal of the 1550 nm, band is extracted. The optical signal of the 1490 nm band is directly transmitted through the first beam splitter and is then incident on a second beam splitter. The second beam splitter reflects the optical signal of the 1490 nm band by 90 degrees, with the result that the optical signal of the 1490 nm band is incident on the photo diode module for the 1490 nm band, whereby the optical signal of the 1490 nm band is extracted. On the other hand, an upstream optical signal of 1310 nm band emitted from the TO type laser diode module is directly transmitted through the first and second beam splitter, and is optically coupled with the optical fiber, whereby the signal is upstream-transmitted. In this way, the three TO type optical modules and the two beam splitters are needed to divide the upstream optical signal of the 1310 nm band, the downstream optical signal of the 1490 nm band, and the downstream optical signal of the 1550 nm band, and a housing is further needed to fix the three TO type optical modules and the two beam splitters, with the result that the structure is very complicated, and a large number of parts are needed, whereby the manufacturing costs are high.
The BiDi module for bidirectional communication or the triplexer module is a technology that is capable of simultaneously transmitting and receiving several optical signals having different wavelengths using a single optical fiber. Moreover, there has been recently adopted a DWDM system that is capable of transmitting optical signals having a very narrow wavelength spacing using a single optical fiber. This DWDM is a system that divides laser light into narrow several nm wavelength bands. In International Telecommunications Union-Telecommunication Standardization Sector (ITU-T), establishing international standards for communication equipment and communication system, laser lights of specific frequencies having a spacing of approximately 100 GHz are established as DWDM. When such allowed frequencies are converted into wavelengths, the wavelength spacing becomes approximately 0.8 nm. Consequently, it is required for the laser light used in the DWDM to have a very narrow emission line width. In addition, it is required that the wavelength of the laser light be stable even irrespective of various changes of external working environment. A distributed feedback laser diode (DFB-LD) is used as laser having a very narrow emission line width. Generally, for a well-manufactured DFB-LD, −20 dB (decibel) line width is within 0.2 nm, which is much less than the wavelength spacing established by ITU-T, and therefore, signal overlapping does not occur during DWDM communication. For a semiconductor laser diode, however, an internal refractive index thereof changes depending upon temperature and injection current density, with the result that the output wavelength changes. Generally, DFB-LD has an emission wavelength change of approximately 0.09 nm with respect to a temperature change of 1° C. That is, when the same DFB laser diode is used at a condition having a temperature difference of approximately 9° C., an emission wavelength band is shifted from an ITU-T grid having an original wavelength band to a neighboring grid.
In order to solve the above-mentioned problem, there has been developed a laser diode package having a thermoelectric device to uniformly maintain the operating temperature environment of a laser diode installed therein. However, the emission wavelength of the DFB laser diode is affected not only by the working environment temperature of a laser diode chip but also by several other factors, including drive current of the laser diode chip. For this reason, there is being required a method for directly monitoring the wavelength change of the laser diode chip, instead of passively adjusting the wavelength of the laser diode to uniformly maintain the drive current of the laser diode chip.
A function to monitor the emission wavelength of the laser diode to uniformly maintain the wavelength of the laser diode is referred to as wavelength locking. The wavelength locking function may be carried out by a method for monitoring diffraction by a diffraction grating outside the laser diode module to check a wavelength and changing the drive temperature of the laser diode based on the checked wavelength. However, this method is very troublesome because the volume of the structure greatly increases. Accordingly, there have been developed methods for directly monitoring the wavelength of the laser diode chip in the laser diode module and changing the working temperature of the laser diode chip to offset the change of the wavelength.
An edge emitting semiconductor laser diode chip, which is a typical example of a semiconductor laser diode chip having a power of a few milliwatts or more, emits laser light from opposite edges of the chip in opposite directions. The intensity of light emitted from the edge of the laser diode chip may be changed by adjusting reflexibility at the opposite edges. Generally, a side from which light is emitted strongly is referred to as a front side of the semiconductor laser chip, and a side from which light is emitted weakly is referred to as a back side of the semiconductor laser chip. Generally, the light emitted strongly from the front side of the laser diode chip is used to transmit a signal for communication, and the light emitted weakly from the back side of the laser diode chip is incident on a photo diode chip installed in the module to monitor the operation state of the laser diode chip.
FIG. 10 is a conceptional view illustrating a wavelength locking laser diode module that monitors the fluctuation of emission wavelength of a laser diode chip in such a conventional laser diode module.
As shown in FIG. 10, laser light emitted from the back side of a laser diode chip 20 installed in a package housing 10 passes through a lens 70, with the result that the laser light is changed into parallel light. A predetermined portion of the intensity of the laser light is directly transmitted, and the remaining portion of the intensity of the laser light passes through a beam splitter 30, by which the advance direction of the laser light is changed 90 degrees, with the result that the laser light is divided into two laser light components having two different advance directions. One of the laser light components directly advances, and the intensity thereof is detected by a photo diode chip 40, which is a light receiving device for monitoring. The other laser light component passes through a filter 50 having wavelength selectivity and is then incident on a photo diode chip 60. On the other hand, laser light emitted from the front side of the laser diode chip 20 passes through a lens 80 and is then condensed to an optical fiber 90 outside the package housing 10.
In the above-described structure, the optical current of the photo diode chip 40 is not changed by the wavelength change of less than a few nm of the laser light for the laser light passing through the beam splitter 30 and directly incident on the photo diode chip 40; however, the optical current property of the photo diode chip 60 is sensitively changed even by the minute wavelength change of less than of a few nm, due to the property of the wavelength-selectivity filter 50, for the laser light passing through the beam splitter 30 and passing through the wavelength-selectivity filter 50. Consequently, the wavelength change of the laser light may be measured by comparing the intensity of the laser light divided into two components after passing through the beam splitter 30. In the conventional method, however, the beam splitter 30 has a size of 1.0 nm×1.0 nm×0.5 nm or more. That is, the size of the beam splitter 30 is very large. Also, the two built-in photo diode chips 40 and 60 are disposed in such a manner that the photo diode chip 40 lies at right angles to the photo diode chip 60, with the result that the area of the module increases. Accordingly, the laser diode module having the wavelength locking function is implemented using a butterfly package housing, which is a package housing the volume of which is greater than those of the other optical communication laser diode housings. Also, in FIG. 10, the installation angle of the rectangular parallelepiped beam splitter 30 in the horizontal direction at the time of installing the beam splitter 30 is adjusted according to installation accuracy, and the direction of light reflected by the beam splitter 30 is changed by the installation angle of the beam splitter 30. Consequently, it is required to very accurately install the beam splitter 30 and the respective photo diode chips 40 and 60. However, this process is difficult and troublesome.
In a present situation in which optical communication parts are implemented in the form of a very small-sized transceiver, such as small form factor (SFF) or small form factor pluggable (SFP), a butterfly package is too large to install in such a small-sized transceiver. Consequently, a DWDM laser diode package housing, which is applied to the transceiver, such as SFF or SFP, which is being presently commercialized, is implemented in a mini-flat or mini-DIL type structure. Such a laser diode package housing has an internal volume insufficient to have two photo diode chips having a wavelength locking function installed therein. For this reason, the present mini-DIL or mini-flat type DWDM package housing is implemented in a structure to uniformly maintain the temperature of the laser diode chip without the wavelength locking function. Due to the omission of the wavelength locking function, the mini-DIL or mini-flat type package adopts a passive temperature stabilization method, which does not actively stabilize the wavelength of the laser diode chip but adjusts only the working temperature of the laser diode chip, with the result that accurate wavelength stabilization is not possible. Therefore, there is a high necessity for a novel package having a wavelength locking function although the size of the package is small sufficient to install in a subminiature transceiver, such as SFF or SFP.
Also, an optical transceiver, manufactured in the form of a mini-flat or mini-DIL type package housing, includes a upstream optical transmitter for an upstream optical signal and a downstream optical receiver for a downstream optical signal, which are manufactured separately in the form of a package housing. The upstream optical transmitter and the downstream optical receiver are installed in the transceiver, such as SFF or SFP. Consequently, two strands of optical fiber are needed to use such an optical transceiver. In recent years, however, there has been widely adopted a bidirectional optical communication module, i.e., a bidirectional optical transmission module, which is capable of transmitting and receiving an optical signal using a single optical fiber. Consequently, there is a high necessity to develop a subminiature laser diode package that is capable of monitoring the fluctuation of emission wavelength of a laser diode chip to drive a thermoelectric device built-in in the direction to offset the change of the emission wavelength to change of the drive temperature of the laser diode chip, thereby stabilizing the emission wavelength of the laser diode chip and achieving bidirectional communication. Up to now, no conventional products, capable of performing bidirectional communication, implemented in the form of a DWDM optical module having a wavelength locking function, have not been invented or come onto the market.
In the edge emitting semiconductor laser diode, as previously described, laser light having intensity proportional to edge transmissivity is emitted from the opposite edges of the laser diode chip. However, this discussion is appropriate when the reflexibility at the opposite edges exceeds a few % or so. If the reflexibility at one edge is 0.1% or less, and the reflexibility at the other edge is several tens of % or so, i.e., if the difference in reflexibility between the edges is large, the energy ratio of light emitted from the opposite edges of the laser diode chip changes depending upon the state of current injection to the laser diode chip. A representative laser diode chip exhibiting this property may be a reflective semiconductor optical amplifier. The front side of the reflective semiconductor optical amplifier generally has a reflexibility of 0.1% or less, whereas the back side of the reflective semiconductor optical amplifier generally has a reflexibility of several tens of % or more. In this case, the intensity of the laser light detected at the back side of the laser diode chip does not represent that of the laser light emitted from the front side of the laser diode chip. For this reason, a method for coupling the laser light emitted from the front side of the laser diode with the optical fiber, dividing some energy of the light coupled with the optical fiber at a predetermined ratio through the optical fiber or an optical distributor manufactured in a waveguide structure, and making the divided optical signal to be incident on the photo diode to monitor the operation state of the laser diode chip is adopted instead of a technology for disposing the photo diode chip to monitor the laser diode chip at the back side of the chip as in the conventional laser diode module. However, the method for disposing the optical distributor at the front side of the laser diode module to monitor the operation state of the laser diode chip is very complicated and, in addition, cost-consuming.
It is a first object of the present invention to provide a self-standing parallel plate beam splitter easy to manufacture and easy to install in a laser diode package, and a method for manufacturing the same.
It is a second object of the present invention to provide a laser diode package structure that is capable of performing bidirectional communication using a self-standing parallel plate beam splitter easy to install.
It is a third object of the present invention to provide a laser diode package structure having a triplexer function using a self-standing parallel plate beam splitter easy to install.
It is a fourth object of the present invention to provide a laser diode package structure having a wavelength locking function using a self-standing parallel plate beam splitter easy to install.
It is a fifth object of the present invention to provide a laser diode package structure having a front side monitoring function to monitor the operation state of a laser diode chip using some of laser light emitted from the front side of the laser diode chip.